1. Field of the Invention
The invention relates generally to power supply circuits and more particularly to circuitry for limiting inrush current transients associated with initial turn-on of a switching regulator type power supply.
2. Description of the Prior Arts
Inrush current transients in power supply circuits are generally caused by charging of substantially uncharged capacitive loads upon the initial application of a voltage to the circuit. This period is referred to as circuit turn-on. The transients may be attributed to the charging of the large value storage capacitors typically connected across the output terminals of a rectifier to filter the rectified unregulated voltage. The inrush surge current is limited only by the impedances of the power transformer, rectifying diodes, and series inductance. Since these impedances are usually very low, a turn-on current as high as ten times the steady-state current may be drawn by the power supply. This inrush surge may cause an undesired voltage drop on the AC power lines.
Techniques presently utilized to limit current transients occuring at the initial application of power to a switching regulator include the placement of a large inductor at the input to the regulator to limit the rate of rise of the input current. Conventionally, a large value of series inductance will provide the requisite impedance to suppress turn-on current transients. A large inductance appears as an open circuit through a suddenly applied voltage, thereby providing a large series impedance to current transients caused by the initial application of the input voltage to the uncharged input filter capacitor. However, a serious disadvantage of a large input inductor is the size and weight to provide sufficient impedance to suppress the inrush current. Further, the energy stored in such an inductor must be dissipated when the switching regulator cycle is turned off to prevent large voltage transients. This requires damper resistors or diodes which complicate the necessary circuitry and increase cost. Therefore, it is advantageous to provide current transient suppression only for the duration of turn-on or the initial current surge, and thereafter to disable the suppression from the rest of the circuitry for circuit operation subsequent to turn-on, as when all capacitive elements have been substantially charged to their full capacity.
An improved prior art technique is shown in FIG. 1. Here a resistor R1 is placed in series with an inductor L1 and a series regulator 10. The coil of a relay K1 is placed in parallel with the load capacitor C1. Contacts X1 of the relay shunt resistor R1. When power switch S1 is closed, applying AC power through transformer windings T1-T3 and diode rectifiers CR1-CR6, inductor L1 and resistor R1 limit the charging current applied to capacitor C1. The voltage across capacitor C1 increases exponentially in accordance with the RLC time constant. When the voltage across capacitor C1 is sufficiently high, the coil of relay K1 will activate contacts X1. Normally open contacts X1 will be closed, thereby short-circuiting resistor R1 and restoring the circuit to full voltage operation. This circuit has the disadvantage, however, that due to the resistance of the series dropping resistor R1, C1 will not be fully charged at the time that relay K1 closes. As a result, when the normally open contacts X1 bypass resistor R1, there is a secondary surge while capacitor C1 charges up to a higher voltage value. This secondary surge can be greater than the initial inrush current transient.
U.S. Pat. No. 4,271,460 discloses a solid-state circuit for switching inrush current impedance into and out of the circuit. This disclosure teaches a suppression impedance of relatively high value connected between a DC supply and a load, a transistorized control circuit for sensing the voltage developed across the load, a firing signal when the voltage exceeds a predetermined potential, and a silicon controlled rectifier (SCR) having its main current path connected in parallel with the suppression impedance, the gate electrode of the silicon controlled rectifier coupled to receive the firing signal for turning on the silicon controlled rectifier so as to bypass the suppression impedance subsequent to application of power to the load. The disadvantage of such a circuit is that if the reference voltage is relatively high with respect to the output voltage of the circuit, there may be a substantial secondary surge, as in the relay circuit of FIG. 1. Further, for relatively low voltage power converters, the voltage drop across the series connected transistor control circuit may be excessive, causing substantial power losses. Moreover, the circuit relies on the series inductance for suppressing the secondary current surge and completing the charging of the load capacitor.